1. Field of the Invention
The present invention relates to a field emission type backlight unit, and more particularly, to an anode panel which provides improved brightness and prevents the temperature of a substrate from increasing, and a field emission type backlight unit having the anode panel.
2. Description of the Related Art
Flat panel display devices can be typically classified into light emitting type display devices and light receiving type display devices. Light emitting type display devices include cathode ray tubes (CRTs), plasma display panels (PDPs), and field emission display (FED) devices, and light receiving type display devices include liquid crystal display (LCD) devices. The LCD devices have the advantages of being lightweight and having low power consumption, but the drawback of being a light receiving type display device. As light receiving type display devices, the LCD devices cannot generate their own light and thus need to use external light to display images, therefore the images cannot be seen in a dark place. In order to address this disadvantage, a backlight unit is installed on the rear surface of the LCD devices.
Conventional backlight units mainly use cold cathode fluorescent lamps (CCFLs) as a line light source and light emitting diodes (LEDs) as a point light source. However, the conventional backlight units have high manufacturing costs due to their structural complexity, and high power consumption due to light reflection and transmittance of lights generated from sides of the backlight units. In particular, achieving uniform brightness of the generated lights become more difficult when the size of LCD devices increases. Recently, in order to address the above disadvantages, field emission type backlight units having a surface light emitting structure have been developed. The field emission type backlight units have lower power consumption than the backlight units that use the conventional CCFLs, and are advantageous in that they have relatively uniform brightness over a wide light emitting region. The field emission type backlight units can be used for illumination.
In a conventional field emission type backlight unit, an anode panel and a cathode panel are spaced apart so as to face each other. A constant interval is maintained between the anode panel and the cathode panel by spacers formed therebetween. Sealing members sealing a space between the anode panel and the cathode panel are formed on edges of the space between the anode panel and the cathode panel. The anode panel includes an upper substrate, an anode formed on a lower surface of the upper substrate and a phosphor layer coated on a lower surface of the anode. The upper substrate may be a glass substrate, and the anode may be formed of a conductive material such as indium tin oxide (ITO) or the like. The cathode panel includes a lower substrate, a cathode formed on an upper surface of the lower substrate and electron emission sources formed on an upper surface of the cathode. The lower substrate may be a glass substrate, and the cathode may be formed of a transparent conductive material or a metal material. The electron emission sources emit electrons due to an electric field generated between the anode and the cathode. Conventionally, a micro tip formed of a metal such as molybdenum is used as the electron emission sources. However, recently, carbon nanotube (CNT) emitters having a good electron emission property have been mainly used as the electron emission sources. A diffuser may be further formed on the anode panel to diffuse visible rays transmitted through the upper substrate.
In the conventional field emission type backlight unit as described above, when a predetermined voltage is applied between the anode and the cathode, electrons are emitted from the electron emission sources, and the emitted electrons are accelerated towards the anode so as to collide with the phosphor layer. Accordingly, the phosphor layer is excited so that visible rays are emitted, and the emitted visible rays are transmitted through the upper substrate and proceed outside. In the conventional field emission type backlight unit, since the visible rays emitted from the phosphor layer are radiated in all directions, the visible rays need to be guided towards an upper part of the upper substrate, on which a liquid crystal panel is disposed, in order to improve light efficiency. In order to do this, a reflective layer (not shown) formed of aluminum may be further formed on the phosphor layer.
The visible rays generated by the phosphor layer-proceed from a lower surface of the upper substrate formed of a glass to the outside of the unit. In this case, a refraction angle of the visible rays emitted to the outside of the unit is considerably greater than an incidence angle because of different refractive indices of the upper substrate and the medium at the outside of unit. Accordingly, part of the visible rays emitted from the phosphor layer deviate from the area over the upper substrate, and thus light efficiency is reduced. In the field emission type backlight, a high voltage should be applied to the anode in order to improve luminous efficiency of the phosphor layer. However, when a high voltage is applied to the anode, the temperature of the upper substrate on which the anode is formed, increases, and accordingly, the upper substrate may be damaged due to thermal expansion as a result of the increased temperature.